Add shared-extensions, frankenphp and zts to build-unix workflow

[N-Silbernagel]

What does this PR do?

This PR adds options for shared-extensions, build-frankenphp and enable-zts to the build-unix workflow. The change enables me to build a (mostly) static frankenphp together with xdebug without installing any tools. I'm hoping this also benefits others.

Needed to add "getenv('SPC_CMD_VAR_PHP_MAKE_EXTRA_LDFLAGS') ?: ''" because "getenv('SPC_CMD_VAR_PHP_MAKE_EXTRA_LDFLAGS')" returns false in gh action where the env var is not set.

Checklist before merging

If your PR involves the changes mentioned below and completed the action, please tick the corresponding option.
If a modification is not involved, please skip it directly.

• If you modified *.php or *.json, run them locally to ensure your changes are valid:

  • [ x ] composer cs-fix
  • [ x ] composer analyse
  • [ x ] composer test
  • [ ] bin/spc dev:sort-config
    This fails, but already does so on commit 4625c6a

• If it's an extension or dependency update, please ensure the following:

  • Add your test combination to src/globals/test-extensions.php.
  • If adding new or fixing bugs, add commit message containing extension test or test extensions to trigger full test suite.

[henderkes]

that's not true, shared extensions work with musl too if you set SPC_TARGET="x86_64-linux-musl -dynamic".

[henderkes]

or on alpine with SPC_MUSL_DYNAMIC=true